Adjust an oven rack to the lowest position and heat the oven to 250 degrees. Using kitchen shears or a chef’s knife, trim the rib bones and any excess fat from the turkey, following the illustration at right. Pat the turkey dry with paper towels and season with salt and pepper.
Heat the oil in a large Dutch oven over medium-high heat until just smoking. Add the turkey, breast side down and scatter the onion, carrot, celery, garlic, thyme, and bay leaf around the turkey. Cook, turning the breast on its sides and stirring the vegetables as needed, until the turkey and vegetables are well browned, 12 to 16 minutes, reducing the heat if the pot begins to scorch. Turn turkey so breast side is facing up.
Off the heat, place a large sheet of foil over the pot and press to seal, then cover tightly with the lid. Transfer the pot to the oven and cook until the thickest part of the breast registers 160 to 165 degrees on an instant-read thermometer, 1 1/2 to 1 3/4 hours.
Remove the pot from the oven. Transfer the turkey to a cutting board, tent loosely with foil, and let rest while making the gravy.
Place the pot with the juices and vegetables over medium-high heat and simmer until almost all of the liquid has evaporated, 15 to 20 minutes. Stir in the flour and cook, stirring constantly, until browned, 2 to 5 minutes. Slowly whisk in the chicken broth, bring to a simmer and cook, stirring often, until the gravy is thickened and measures about 2 1/2 cups, 10 to 15 minutes.
Strain the gravy through a fine-mesh strainer and season with salt and pepper to taste. Carve the turkey and serve, passing the gravy separately.